A fiber-reinforced plastic comprising a reinforcing fiber and a matrix resin exhibits a high specific strength and a high specific elastic modulus and is excellent in mechanical properties as well as has high functional properties such as weather resistance and chemical resistance and, therefore, the fiber-reinforced plastic is receiving a lot of attention also in applications for industry. The fiber-reinforced plastic has being applied to various applications for structural members for aircraft, spacecraft, automobiles, trains, marine vessels, electrical appliances, sports gear and the like, and the demand for the fiber-reinforced plastic is also increasing year after year.
Especially, a thermosetting resin has been used as a matrix for a long time because the viscosity of the thermosetting resin is lower than that of a thermoplastic resin and, therefore, the thermosetting resin is impregnated into fibers easier compared to the thermoplastic resin. Among the processes of producing these fiber-reinforced plastics, there are an autoclave molding method and a press molding method as high-quality molding methods with less voids and the like. The former has had a problem that the molding equipment is large in size and initial costs are too much, and the latter has had a problem that a double-sided mold is required and the size of the member which is capable of being pressurized is limited.
As such, in recent years, there has been an attempt to mold a fiber-reinforced plastic by an out-of-autoclave molding method which uses a vacuum pump and an oven (for example, U.S. Pat. No. 6,139,942). Research on an atmospheric pressure molding method has been progressing as follows. The atmospheric pressure molding method comprises using a partially impregnated prepreg which has a reinforcing fiber partially impregnated with a matrix resin, and evacuating internal air and volatile components from the prepreg with a vacuum pump through an unimpregnated part among the reinforcing fibers within the prepreg, and finally, the resin is impregnated into the reinforcing fibers in the partially impregnated prepreg. This method has advantages that relatively reduced initial costs are required because the molding apparatus is an oven and large size members are easily molded because the method employs vacuum pressurization by using a single-sided mold.
On the other hand, a process of producing a reinforced plastic from a fiber-reinforced material containing a reinforcing fiber impregnated with a thermosetting resin composition by using a vacuum pump and an oven has a problem that the impregnation time and the molding cycle time are longer than those of the autoclave molding method and the press molding method because the pressure difference which promotes the resin impregnation is 1 atm or lower, and an additional problem that voids tend to remain easily and the defective rate is high. In addition, that process of producing the reinforced plastic by using the vacuum pump and the oven has problems that the time for raising the temperature is long and the molding cycle time is long especially in large size members because heat is transferred from air under an atmospheric pressure in that process, in contrast to the autoclave molding method and the press molding method in which heat is transferred from a high pressure gas and a metal having good thermal conductivity, respectively and, therefore, the temperature of the fiber-reinforced plastic can be rapidly raised to a desired temperature. As a result, that process of producing the reinforced plastic by using the vacuum pump and the oven has a further problem that the productivity is reduced compared to the autoclave molding method and the press molding method.
Therefore, based on the above-described background, it could be helpful to provide a process of producing a fiber-reinforced plastic capable of employing atmospheric pressure molding and capable of producing a high-quality fiber-reinforced plastic with good yield in a short molding cycle time.